One or more embodiments relate to a nonvolatile memory device and, more particularly, to a method of reducing errors resulting from interference when a read operation is performed in a nonvolatile memory device.
In recent years, there is an increasing demand for nonvolatile memory devices, which can be electrically programmed and erased and which do not require the refresh function of rewriting data at specific intervals.
The nonvolatile memory cell of the nonvolatile memory device can be electrically programmed and erased, and the program and erase operations are performed using the threshold voltage of the cell, which varies when electrons are moved by a strong electric field applied to the thin oxide layer of the cell.
In such a method of programming a nonvolatile memory device, interference phenomena may be generated as a result of a shift in the threshold voltages of neighboring cells. In particular, in a method of programming a multi-level cell (MLC) capable of storing data of 2 bits or more in one cell, malfunction resulting from such interference phenomena may happen because a margin between the threshold voltages in each state is narrowed. Accordingly, interference resulting from the program operations of neighboring memory cells needs to be minimized. The MLC program method is performed to store information of 2 bits or more in one cell. In an SLC program method, two different states with different threshold voltages are generated by a program operation. In the MLC program method, information of 2 bits or more is stored by repeatedly performing the program operation.
FIG. 1 is a diagram illustrating an interference phenomenon of a nonvolatile memory device. In FIG. 1, dotted lines show distributions of the threshold voltages when an interference phenomenon is not generated, and solid lines show distributions of the threshold voltages when interference phenomena are generated. Portions of the distributions indicated by a letter ‘A’ show portions in which interference is generated and the threshold voltages overlap each other.
As described above, since data are stored in one cell, the number of threshold voltages Vt distinguishing between the data is increased, and so a gap between the threshold voltages is narrowed. Accordingly, as shown in FIG. 1, an interference phenomenon in which the threshold voltages overlap each other can occur, which leads to an inaccurate data reading when a read operation is performed.